Don't go reserves. If you're gonna join active duty is where it's at. There are alot of great choices that won't have you behind a desk. All depends on what you are interested in.
I was a 12B(Combat Engineer) I believe now they are called 21B. There are generally two types, light and mechanized. I was in an airborne unit so I was light. The mechanized guys use armored vehicles and support mechanized infantry. Combat Engineers generally work closely with the infantry and cross train on infantry tasks often because if you are not doing a combat engineer mission you are doing infatry especially since you're right there with them.
In a nutshell 21B's are trained in explosives(tnt,c4, etc), land mines, booby traps, wire obstacles, etc etc. We clear mine fields, emplace mine and mine fields, destroy things with demo, emplace wire obstacles and stuff like that.
It was a lot of fun and I do miss it. I've been out about three years now and if this police stuff doesn't happen I keep thinking of going back. Good luck.